Monogenic volcanic fields are made up of tens-to-thousands small monogenic eruptive centers. They are frequent in some “complex” geodynamic frameworks, like the Trans- Mexican Volcanic Belt and the Central Kamtchatka Depression, and in some extensional settings such as the Northern Main Ethiopian Rift (NMER). This area is characterized by an oblique en-echèelon system of active faults at the floor of the Rift Valley (the Wonji Faults Belt), which actually accommodates all the extension of the NMER and is the main site of volcanic activity. Here, volcanism is characterized by central volcanoes, eruptive fissures with lengths of several kilometers and small basaltic monogenic scoria cones (MSC, >200) with associated minor lava flows, aligned along the Wonji segments. Fissural products have trachybasaltic composition and are aphyric (P.I. <1-2vol%), whereas MSC are basalts with P.I. of 15-30vol%, with plagioclase phenocrysts as the dominant phase. Mass balance calculations showed a same parental magma for all these products, but different crystallization histories. MSC products derive from the shallow fractionation of ca. 15vol% of crystals (Ol, Cpx, Ox, Opx) plus cumulus of 30-40vol% of plagioclase. Fissural magmas underwent more extensive fractionation (35vol%) of the same mafic phases. We put forward the idea that: 1) fissural trachybasaltic magmas reside at the Moho, where they fractionate mafic phases: when eruption is triggered (probably for tectonic reasons), they rise directly to the surface through fault systems; 2) MSC basaltic products derive from low-pressure crystal fractionation (<10-12 km) of a magma which rose up from the Moho through the lithospheric Wonji faults, later was stored at shallower depths (elongated along the main fault system) and crystallized. In this view, monogenic volcanic fields are generated from a slow ascent (mainly depending from density contrast) from this low-pressure magma reservoir of several small-volume blobs of magma, from which every MSC is generated.
|Titolo:||Toward a new concept for magma ascent dynamics at monogenic volcanic fields: the archetype of the northern Main Ethiopian Rift|
NICOTRA, EUGENIO (Corresponding)
|Data di pubblicazione:||2018|
|Appare nelle tipologie:||4.2 Abstract in Atti di convegno|